BIOKINETIC MODELS FOR INTERNAL DOSIMETRY

Intakes of radionuclides can occur via a number of pathways. In occupational exposure, the main route of intake is by inhalation, although a fraction of any material deposited in the respiratory system will be transferred to the throat and swallowed, giving the opportimity for absorption in the gastrointestinal tract. Intakes by direct ingestion may occm, as, for some radionuchdes, may absorption through the intact skin. Damage to the skin by cuts or other wounds can also result in intakes of radionuclides (Fig. 2(a)). 

(a) Routes of intake, transfers and excretion (based on [8]) (b) general model used to represent the kinetics of radionuclides in body compartments (exceptions are noted in the metabolic data for individual elements) (based on [8, 9]). 

In the BSS [2] and in the 1990 Recommendations of the ICRP [6], the approach to calculating the committed effective dose is based on that used for the calculation of committed effective dose equivalent, although as a result of improved information on the late effects of radiation on the tissues of the body some changes have been made to the values of tissue weighting factors and a greater munber of tissues now have specified weighting factors (see Table A-in). 

Details of a new model of the human respiratoiy tract for radiological protection purposes have been issued by the ICRP [19] the main features of this model are described below. This model was used in the ealculation of the inhalation dose coefficients given in the BSS [2]. As in the earher model [29], deposition and clearance are treated separately. 

Respiratory tract regions defined in the new ICRP model [19], The extrathoracic (ET) airways are divided into Ef, the anterior nasal passage, and ET2, which consists of the posterior nasal and oral passages, the pharynx and larynx. The thoracic regions are bronchial (BB trachea, and main bronchi), bronchiolar (bb bronchioles) and alveolar-interstitial (AT the gas exchange region). Lymphatic tissue is associated with the extrathoracic and thoracic airways (LN j and LNj.jj respectively). 

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